Portable VirtualBox kernel driver error

I'm using PortableVirtualBox for about a year with no problems. But from two weeks now -without upgrading or updating anything on Virtualbox or anything related on my PC- after running any of my VMs for a while then I cannot start any other VM, because VB pops up this error

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VirtualBox - Runtime Error
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Cannot access the kernel driver!
Make sure the kernel module has been loaded successfully.
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Abort   
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If I check the VB kernel driver with ServiWin the driver (VBoxDrv) shows up as "Stopping...", but never stops totally.

• I tried with NET STOP VBoxDrv (as administrator), but it says that the service is starting or stopping


• Tried the same running as System with PSEXEC -s, same result.


• Reinstalled PortableVB and the VB runtimes, no luck


• Try to kill the program using sc queryex vboxdrv, but the status is STOP_PENDING and the PID is 0, then I cannot find the process to kill


• Checked memory with Memtest86 disk with MHDD, did a Prime95 torture test to find posible hardware errors: no problems there


• If I reboot the PC, then I can use VB but the problem strikes again after starting a couple of VMs (or restarting the same VM after using it for a while)

My configuration:

• PortableVirtualBox 3.3.6.1 (Launcher)


• VirtualBox 4.0.10.r72479


• VB USB and network drivers installed


• Win7 Pro-64 Host


• AMD Athlon II X3 450 / 8 Gby RAM

Any ideas to fix the initial error or force the VB driver to restart without rebooting?